The basic ingredients
Milk: I use cow’s milk and make a half gallon at a time. Skim milk makes the firmest yogurt, while milk with higher fat content makes creamier yogurt. Goat’s milk can also be used but turns out a thinner yogurt. Ditto for soy milk, plus some brands will result in a smoother consistency than others. (After taking this photo, I tried Silk and now prefer it over West Soy.)
Culture: I’ve had the most reliable results with freeze-dried cultures. Store bought yogurt also works as a starter culture, as long as it has live active cultures in it. Once you’ve made your yogurt, you can use it to culture your next batch. After several rounds of re-culturing, however, you’ll need a new starter culture.
Thickeners: For a thicker yogurt or a yogurt that doesn’t weep whey, stir dried milk powder, starch thickeners (I like arrowroot), or softened gelatin or agar powder into the milk as you are heating it. Adding a thickener may be particularly desirable with goat’s milk and soy milk yogurts.
Flavorings: You can sweeten your milk and/or flavor it as you heat it: with a shot of espresso (pictured), a little almond extract, or the scrapings of a vanilla bean. If your soy milk doesn’t already contain some sugar, you’ll need to add a couple of teaspoons to give the cultures something to feed on.
Fruit yogurt: Stir in cooked and sweetened fruit or fruit preserves after the yogurt has been made. Or, put the preserves at the bottom of the container in which the yogurt will incubate.
Heat milk to 185 to 195°F
Soy milk does not have to be heated this high. You just need to warm the milk enough to get any thickeners going— try 3 teaspoons of arrowroot starch per quart of soy milk, heated between 150 and 160°F. If you aren’t using a thickener, then just warm the milk to 110°F, and move to the next step.
Cool the milk to 110°F
Mix in the culture
Soy milk users: A store-bought soy milk yogurt such as Whole Soy is a non-dairy culture option.
Let the yogurt set for about 8 hours
These days, I use an insulated container designed for yogurt-making (it's called a "Yogotherm"). But you don’t need special equipment. As you'll see in the next slide...
Making yogurt without an actual yogurt maker
You can use a cooler in much the same way. Also, I haven’t tried it, but an oven with a "proof mode" or a pilot light should work, too, but may take a little longer, given the slightly lower temperature.